The present invention relates to a process for eliminating chlorine atoms from chlorofluorocarbons (CFCs) obtaining the corresponding fluorocarbons (FCs). The Montreal Treaty has banned the chlorofluorocarbon production, commercialization and dispersion in the atmosphere and therefore they cannot be industrially used any longer. These compounds can be disposed of by combustion in special incinerators, or can be transformed into harmless products for the ozone layer by using processes of the prior art. Specifically the processes are the following: the chlorofluorocarbon reaction with anhydrours hydrofluoric acid on catalyst, with formation of fluorocarbons (FCs), or the reaction with elementary hydrogen on platinum or palladium catalyst, which leads to the formation of the corresponding hydrofluorocarbons (HFCs).
Both these processes require specific plants since, as known, HF is highly toxic, hydrogen is flammable; the hydrochloric acid which develops in each of the two reactions is toxic and corrosive.
The need was therefore felt to have available processes in order to obtain the CFC conversion into industrially usable compounds.
An object of the present invention is a gaseous process for elimianting chlorine atoms from a chlorofluorocarbon (CFC) having formula CnFxCly, wherein n is an integer from 1 to 3 and x+y=2n+2, x being an integer in the range 1-7, preferably n is 1 or 2 and x is in the range 1-5, wherein said compound is reacted with a hydrofluorocarbon of formula Cnxe2x80x2Fxxe2x80x2Hyxe2x80x2 wherein nxe2x80x2, xxe2x80x2, yxe2x80x2, equal to or different respectively from n, x, y, have the same meaning of n, x, y, in the presence of a fluorination catalyst in solid phase at temperatures in the range 200xc2x0 C.-400xc2x0 C., preferably 250xc2x0 C.-320xc2x0 C.
With the process of the invention it has surprisingly been found that a halogen exchange between CFC and HFC takes place, wherein CFC acquires fluorine and changes at last into FC while HFC acquires chlorine and changes into HCFC. There is no formation of other undesired by-products.
Starting from a CFC containing only one chlorine atom the corresponding FC is obtained. When the starting chlorofluorocarbon contains more than one chlorine atom, the process is repeated the necessary times until the corresponding fluorocarbon is obtained.
The HCFC compound obtained from HFC according to the reaction of the present invention can optionally be used as such, or recycled in the prior art processes for the HCFC/HFC production.
Therefore the process according to the present invention allows to obtain only compounds which have an industrial interest.
Said reaction is very selective and the amounts of organic by-products, and also of obtained hydrohalogenic acids (HF and/or HCl), are very reduced.
The fluorination catalyst of the invention is well known in the prior art, preferably a trivalent chromium compound, for example chromium oxide, optionally supported, can be mentioned. See the patents WO 95/16654 or U.S. Pat. No. 5,345,014.
The contact time with the catalyst, measured as the ratio between the catalyst volume and that of the gas flow at the working temperature and pressure, is not critical: long contact times lead to high conversions, while short contact times involve a higher flow of reacting gases, the catalyst volume being equal, and a lower conversion. There is therefore a balance, which is found by the skilled in the art, between the contact time and the conversion obtainable at each step, so as to optimize the reactor productivity. Generally the contact times range from 1 to 30 seconds.
The amount by moles of CFC and HFC in the mixture depends on the specific mixture under reaction.
CFCs are preferably selected from CFC 114 and CFC 115, HFCs from HFC 125 and 134a. The percent by moles of CFC 114 in the mixture HFC 125 (CHF2CF3) and CFC 114 (CClF2xe2x80x94CClF2) can for example range between 5% and 25%, while that of 134a in the mixture HFC 134a (CF3CH2F) and CFC 115 is at most of 15%.
The skilled in the art is easily capable to experimentally determine the preferred molar ratios between HFC and CFC.
The working pressure is not critical, but preferably one operates in the range 1-10 bar.
The reaction is carried out by flowing the CFC/HFC mixture, optionally diluted with an inert gas, through the catalyst in a fixed or fluidized bed. When the catalyst is in a fluidized bed the catalyst particles have sizes suitable for this process.
The fluorination catalyst can be supported, and the support is preferably aluminum fluoride AlF3 obtainable by alumina fluorination and having a fluorine content not lower than 90%, preferably not lower than 95%, with respect to the stoichiometric.
Generally the aluminum fluoride is mainly constituted by gamma phase, as described in the patent FR 1,383,927, and has a surface area generally in the range 25-35 m2/g. When the catalyst is used in a fluidized bed, the support must have the granulometry suitable for this kind of reactor, as it is well known to the skilled in the prior art.
When a trivalent chromium compound is used as fluorination catalyst, the chromium amount in the supported catalyst is in the range 5-15% by weight, preferably 10-15%, determined as metal amount with respect to the weight of the finished supported catalyst.
The supported catalyst is preferably prepared by impregnation of the support with an aqueous solution of a soluble salt.
The impregnation of the support can be carried out by any method known in the art, for example by the method known as xe2x80x9cdry impregnationxe2x80x9d.
According to this method the impregnation is effected by pouring on the support, in sequence, aliquots of an impregnating solution, such that the total volume is not higher than the volume of the aluminum fluoride pores. The solution for the impregnation is prepared by dissolving in water the required amounts of the corresponding salts, preferably chlorides, of the desired metal, preferably trivalent chromium. The solution is poured in aliquots on the support, drying at 110xc2x0 C. for some hours after each addition, to evaporate water from the support pores.
The unsupported catalyst is prepared by methods known in the art, for example by precipitation of a metal soluble salt, preferably of trivalent chromium as described in U.S. Pat. No. 5,345,014.
Before use the catalyst is activated by calcining for 4-8 hours in a current of inert gas, at the temperature of 400xc2x0 C., and then treating it at 360xc2x0 C. with anhydrous HF for a time comprised between 12 and 24 hours. The operation can be carried out in the reactor used for the process of the invention.
During the use the catalyst undergoes a slow deactivation, due to the deposit of organic substance; it can be regenerated by treating it with air at a temperature in the range 300xc2x0 C.-400xc2x0 C. for 4-8 hours and then with anhydrous HF at 360xc2x0 C. for 12-24 hours.
It has been found that in the process of the invention the catalyst duration is very high.